Project Sign was an official U.S. government study of unidentified flying objects (UFOs) undertaken by the United States Air Force and active for most of 1948.
Project Sign's final report, published in early 1949, stated that while some UFOs appeared to represent actual aircraft, there was not enough data to determine their origin.
Project Sign was first disclosed to the public in 1956 via the book The Report on Unidentified Flying Objects by retired Air Force Captain Edward J. Ruppelt. The full files for Sign were declassified in 1961.
References
Project Sign Wikipedia(Text) CC BY-SA